On 5/18/14, 2:17 AM, Roger (K8RI) on TT wrote:
On 5/18/2014 12:44 AM, Jim Lux wrote:
On 5/17/14, 9:13 PM, Roger (K8RI) on TT wrote:
On 5/17/2014 11:27 PM, Jim Lux wrote:
On 5/17/14, 6:11 PM, Drax Felton wrote:
What is the conductance per inch of concrete in mho's ?
http://www.k1ttt.net/technote/concrete.html
10-50 mS/m
Shouldn't the medium be in cubic units?
No.. it works out right if you do the dimensional analysis..
Not if you do the math right.
Resistivity is in ohm-m, conductivity in 1/(ohm-m) or S/m (since S is
1/ohms)
Resistivity is not ohms per m, it's usually given as ohms per cubic
centimeter.
Resistance usually is dimensionless, unless it's bulk and then it's so
many ohms per unit length
Resistance isn't actually dimensionless, but we can consider it so here.
Resitivity (rho in most equations)
The SI unit of electrical resistivity is the ohm⋅metre (Ω⋅m)
So, to compute the resistance of a bar of material that has length L (in
meters) and cross sectional area A (in square meters), you'd calculate as:
R = rho (ohm * meters) * L (meters) / A (meters*meters)
reducing units, you see that you wind up with (ohms * meters *
meters)/(meters * meters), so the result is in ohms, which is what you
would expect.
Conductivity (sigma) is in Siemens per meter (S/m). Just as for
resistance, to calculate the conductance you follow a similar equation.
Doubling the length halves the conductance; doubling the area doubles
the conductance so
G = sigma (Siemens/meter) * Area (meters * meters) / Length (meters)
Again, the length units cancel, so we're left with G in Siemens
You are confusing resistance with resistivity. You are losing a unit
somewhere because resistivity is ohms per cubic meter or more often ohms
per cubic centimeter. Look up the dictionary definition. Resistance is
between two points. Resistivity takes into account the volume as the
units are .
Resistivity is in ohms per cubic centimeter. I worked with developing
standards for liquids and circular solids With a Silicon production
company for many years.
The dictionary definition: for resistivity takes into account both
area and length. IE: volume as in resistivity ohms ^3 cm. Had a devil
of a time getting the techs to stop confusing the two. The easiest was
to give them a probe consisting of 2, 1cm square plates, 1 cm apart,
then thy had no conversions.
Resistance = Length * resistivity/cross sectional area.
No. Again that is resistivity
Not according to the SI definition.. Resistance is measured in ohms, etc.
I think you might be conflating "how it's measured" vs "how it's defined".
If I take a bar of material, and connect my resistance meter to the ends
(assuming I can contact the entire end simultaneously, e.g. if I had a
bar of carbon with silver plated ends) then everything works out.
so to get ohms as resistance, you need resistivity to be ohms * length
(because Length/area is 1/length)
Likewise, conductivity is = Area * conductivity/length, so it needs to
be Siemens/meter
Conductivity is the reciprocal of resistivity, not resistance. Please
stop working the formulas and just look up the definitions
http://en.wikipedia.org/wiki/Electrical_conductivity
Electrical resistivity (also known as resistivity, specific electrical
resistance, or volume resistivity) quantifies how strongly a given
material opposes the flow of electric current. A low resistivity
indicates a material that readily allows the movement of electric
charge. Resistivity is commonly represented by the Greek letter ρ (rho).
The SI unit of electrical resistivity is the ohm⋅metre (Ω⋅m)[1][2][3]
although other units like ohm⋅centimetre (Ω⋅cm) are also in use. As an
example, if a 1 m × 1 m × 1 m solid cube of material has sheet contacts
on two opposite faces, and the resistance between these contacts is 1 Ω,
then the resistivity of the material is 1 Ω⋅m.
Electrical conductivity or specific conductance is the reciprocal of
electrical resistivity, and measures a material's ability to conduct an
electric current. It is commonly represented by the Greek letter σ
(sigma), but κ (kappa) (especially in electrical engineering) or γ
(gamma) are also occasionally used. Its SI unit is siemens per metre
(S/m) and CGSE unit is reciprocal second (s−1).
73
Roger (K8RI)
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